In the Universal Mobile Telecommunications System (UMTS) network, the specifications of long term evolution (LTE) have been drafted for the purpose of further increasing high speed data rates, providing lower delays and so on (see non-patent literature 1). Also, the specifications of LTE-A (also referred to as LTE-advanced, LTE Rel. 10, 11 or 12) have been drafted for further broadbandization and increased speed beyond LTE (also referred to as LTE Rel. 8 or 9), and successor systems of LTE (also referred to as, for example, Future Radio Access (FRA), fifth generation mobile communication system (5G), LTE Rel. 13 and so on) are under study.
Also, the specifications of LTE-A (also referred to as “LTE-advanced,” “LTE Rel. 10,” “LTE Rel. 11” or “LTE Rel. 12”) have been drafted for further broadbandization and increased speed beyond LTE (also referred to as LTE Rel. 8 or 9), and successor systems of LTE (also referred to as, for example, “FRA” (Future Radio Access), “5G” (fifth generation mobile communication system), “LTE Rel. 13” and so on) are under study. Carriers that constitute the fundamental units in carrier aggregation (CA) are referred to as “component carriers” (CCs), and are equivalent to the system band of LTE Rel. 8.
When CA is used, in a user terminal (User Equipment (UE)), a primary cell (Primary Cell (Pcell)), which is a cell with high reliability to ensure connectivity, and a secondary cell (Secondary Cell (Scell)) which is an adjunct cell, are configured.
The UE can first connect to the PCell and add the SCell if necessary. PCell is a single cell (standalone cell) that supports Radio Link Monitoring (RLM), Semi-Persistent Scheduling (SPS), and the like. SCell is a cell that is configured in UE in addition to PCell.
SCell is added and deleted by Radio Resource Control (RRC) signaling. SCell is in a deactivated state immediately after being added to the user terminal, and can only perform communication (scheduling) after being activated.
On the other hand, in LTE Rel. 12, dual connectivity (DC) has also been introduced where multiple cell groups (CG) formed by different radio base stations are configured in the user terminal. Each cell group consists of at least one cell (CC). In DC, since multiple CCs of different radio base stations are integrated, DC is also called inter-eNB CA.
Also, the specifications of LTE Rel. 8 to 12 have been drafted assuming exclusive operations in frequency bands that are licensed to operators (licensed bands). As licensed bands, for example, the 800 MHz, 2 GHz and/or 1.7 GHz bands are used. Meanwhile, in LTE of Rel. 13 and later versions, operation in frequency bands where license is not required (unlicensed bands) is also a target of study. For unlicensed bands, for example, the 2.4 GHz and/or the 5 GHz band are used as in Wi-Fi (registered trademark).
Although carrier aggregation (license-assisted access (LAA)) between licensed bands and unlicensed bands is placed under study in Rel. 13 LTE, there is a possibility that, in the future, dual connectivity (DC) and unlicensed-band stand-alone will becomes targets of study as well.
In LTE Rel. 8 to 12, Hybrid Automatic Repeat reQuest (HARQ) is used for retransmission control. In HARQ, the user terminal (or the radio base station) feeds back the delivery acknowledgment signal (also referred to as HARQ-ACK, ACK/NACK, A/N, etc.) for the data in accordance with the data reception result at a predetermined timing. The radio base station (or user terminal) controls data retransmission based on the feedback HARQ-ACK.